In the publication, Proceedings of the Drive Conference: "Advanced Telematics in Road Transport" Feb 4-6, 1991, Vol 1, it is proposed in the context of the PAMELA project that a data exchange be carried out in the microwave range with an active and semi-passive transponder. Data are transmitted from a beacon unit to the transmission and reception part of the vehicle unit in what is known as the downlink mode, by means of amplitude-modulated signals. The data transmission from the vehicle to the beacon is done in the uplink mode by the transponder principle. The beacon sends an unmodulated carrier signal, which is received by the vehicle unit. This received signal is frequency-modulated with the data to be transmitted and is received again by the beacon unit. Since the beacon units have only a short range within which reliable data transmission is possible, it is necessary especially on multiple-lane roads or at intersections to set up a plurality of beacons with corresponding transmitters. Yet there is then at least partial overlap in the areas illuminated by these beacons.
If the beacons, that is, the stationary units, were then to transmit at the same frequency, an interference field with highly location-dependent field intensities would be generated. Ground and vehicle reflections, however, also lead to major location-dependent changes in the reception power, however, so that overall the data transmission between the vehicle and the stationary unit and vice versa does not appear reliable enough.